1. Field of the Invention
This invention relates to a gas detection device for a mixed gas, which can determine gas information including kinds and concentrations of gas components in a mixed gas consisting of a plurality of known components.
2. Description of the Prior Art
Semiconductor gas sensors such as tin oxide (SnO.sub.2), iron oxide (Fe.sub.2 O.sub.3), and zinc oxide (ZnO) can detect gas concentrations in the form of intensities of electrical signal as outputs and are handled with ease and are manufactured at a low cost, and thus they have been widely used so far. Their working principle is such that a change in resistance of a semiconductor gas sensor by adsorption of a sample gas onto the active part of the sensor is determined in the form of a voltage output by a detection circuit, one example of which is shown in FIG. 1, where numeral 11 is a fixed resistor, 12 a .alpha.-Fe.sub.2 O.sub.3 based sensor for detecting isobutane (C.sub.4 H.sub.10) and 13 a voltmeter.
Relations between the detected gas concentration and the detected voltage are usually linear in some region, as shown by typical detection characteristics 21 of isobutane (C.sub.4 H.sub.10) obtained by .alpha.-Fe.sub.2 O.sub.3 based sensor in FIG. 2, and thus are well applicable to practical measurement.
More specifically, reference will be made to the prior art, where water vapor concentration and alcohol vapor concentration are separately detected at the same time in a mixed gas of air, water vapor and alcohol vapor, as has been so far thoroughly studied. For example, Japanese Laid-Open patent application No. 80192/75 discloses that a few species of sensor materials are required for simultaneous but independent detection of a plurality of gas components, and also a plurality of sensors are required for detection of only one species of gas component, if no sensor is available for detecting only such a species of gas component. In particular, this patent document discloses that, in the latter where no sensor is available for detection of only one species of gas component, for example, when it is desired to detect a gas component a but no sensor is available for detecting only the gas component a and only a sensor A sensitive to both gas components a and b is available, a sensor sensitive not to the gas component a but to the gas component b is prepared to ascertain that there is no gas component b, thereby enabling the sensor A to detect the gas component a. The said Japanese Laid-open patent application discloses that changes in resistances of lanthanum-nickel oxide material (LaNiO.sub.3) and magnetite material (Fe.sub.3 O.sub.4) are calibrated against mixed gases having known concentrations in advance, thereby detecting individual gases in a mixed gas of air, water vapor and alcohol vapor having unknown concentrations.
However, the said gas sensor of the prior art type fails to satisfy the social needs for separation and quantitative determination of a mixed gas, as recently encountered in detection of automobile exhaust gas and chemical plant leak gas. For example, as shown by the detection characteristics of zinc oxide (ZnO) based sensor doped with palladium as a H.sub.2 gas sensor in FIG. 3, such a sensor detects the output voltage by H.sub.2 (characteristic curve 31 in FIG. 3), as well as that by carbon monoxide (CO) (characteristic curve 32 in FIG. 3) and that by hydrocarbon gas (propane gas, characteristic curve 33 in FIG. 3) at the same time, and thus the detection accuracy of H.sub.2 by such a sensor is considerably lowered. This seems due to adsorption of other gas components than H.sub.2 giving a change to H.sub.2 adsorption, and this phenomenon usually appears in any gas sensor material as a large disadvantage of the conventional semiconductor gas sensor.